Hearing impairment can be characterized according to its physiological source. There are two general categories of hearing impairment, conductive and sensorineural. Conductive hearing impairment results from diseases or disorders that limit the translation of acoustic sound as vibrational energy through the external and/or middle ear structures. Approximately 1% of the human population is estimated to have ears that have a less than ideal conductive path for acoustic sound. In contrast, sensorineural hearing impairment occurs in the inner ear and/or neural pathways. In patients with sensorineural hearing impairment, the external and middle ear function normally (e.g., sound vibrations are transmitted undisturbed through the eardrum and ossicles where fluid waves are created in the cochlea). However, due to damage to the pathway for sound impulses from the hair cells of the inner ear to the auditory nerve and the brain, the inner ear cannot detect the full intensity and quality of the sound. Sometimes conductive hearing loss occurs in combination with sensorineural hearing loss. In other words, there may be damage in the outer or middle ear, and in the inner ear or auditory nerve. When this occurs, the hearing loss is referred to as a mixed hearing loss. Many conditions can disrupt the delicate hearing structures of the middle ear. Common causes of conductive hearing loss include congenital defect, infection (e.g., otitis media), disease (e.g., otosclerosis), blockage of the outer ear, and trauma (e.g., perforated ear drum).
There are several treatment options for patients with middle hear hearing loss. With conventional acoustic hearing aids, sound is detected by a microphone and converted into an electrical signal, which is amplified using amplification circuitry, and transmitted in the form of acoustical energy by a speaker or other type of transducer. Often the acoustical energy delivered by the speaker is detected by the microphone, causing a high-pitched feedback whistle. Moreover, the amplified sound produced by conventional hearing aids normally includes a significant amount of distortion. Some early hearing aids were also equipped with external bone vibrators that would shake the skin and skull in response to sound. The bone vibrators had to be worn in close contact with the skull in order to transduce signal to the inner ear, thereby causing chronic skin irritation in many users. In addition, external bone vibrators were notably inefficient. These drawbacks spurred the development of microsurgical techniques for the treatment of conductive hearing loss. In fact, otologic surgery (e.g., tympanoplasty, ossiculloplasty, implantation of total or partial ossicular replacement prothesis, etc.) has become an accepted treatment for the repair and/or reconstruction of the vibratory structures of the middle ear. However, these types of procedures are complex and are associated with the usual risks related to major surgery. In addition, techniques requiring disarticulation (disconnection) of one or more of the bones of the middle ear deprive the patient of any residual hearing he or she may have had prior to surgery. This places the patient in a worsened position if the implanted device is later found to be ineffective in improving the patient's hearing.
Thus, there remains a need in the art for medical devices and techniques, which provide improved sound perception by individuals with conductive or mixed hearing loss. In particular, there is a need in the art for hearing aids that efficiently transduce acoustic energy to the inner ear without risk of destroying a patient's residual hearing. The present invention provides hearing devices that provide suitable stimulation to structures of the inner ear resulting in superior hearing correction, and which can be partially implanted in a simple outpatient procedure.